Visual Magnocellular Function in Perceptual Disorders

Developmental disorders such as autism spectrum disorders (ASD), dyslexia, schizophrenia and dyscalculia have also been reported to show abnormal visual perception. Central to the four disorders are observations of altered global/local perception, motion sensation and grouping that are suggestive of a magnocellular abnormality(s). Such psychophysical observations do not easily yield neurophysiological mechanisms that can explain the altered perception/vision. Nonlinear visual evoked potentials have allowed the separation of magnocellular (M) and parvocellular (P) contributions to the VEP (Klistorner et al., 1997). Using these tools we compare the patterns of abnormality in groups with visual disorders. The second order kernel responses of the VEP in autistic tendency show interference between P and M nonlinearities at high contrast (Sutherland & Crewther, 2010) resulting in a delay of completion of firing. While afferent latencies of M and P cortical activation are not different in ASD, the delay in completion may allow a revision of the ideas surrounding the “magnocellular advantage” which relate to the alterations observed in global and local perception.